Mangroves are intertidal ecosystems that are particularly vulnerable to climate change. At the low tidal limits of their range, they face swamping by rising sea levels; at the high tidal limits, they face increasing stress from desiccation and high salinity. Facilitation theory may help guide mangrove management and restoration in the face of these threats by suggesting how and when positive intra- and interspecific effects may occur: such effects are predicted in stressed environments such as the intertidal, but have yet to be shown among mangroves. Here, we report the results of a series of experiments at low and high tidal sites examining the effects of mangrove density and species mix on seedling survival and recruitment, and on the ability of mangroves to trap sediment and cause surface elevation change. Increasing density significantly increased the survival of seedlings of two different species at both high and low tidal sites, and enhanced sediment accretion and elevation at the low tidal site. Including Avicennia marina in species mixes enhanced total biomass at a degraded high tidal site. Increasing biomass led to changed microenvironments that allowed the recruitment and survival of different mangrove species, particularly Ceriops tagal .

The importance of mangrove forests in carbon sequestration and coastal protection has been widely acknowledged. Large-scale damage of these forests, caused by hurricanes or clear felling, can enhance vulnerability to erosion, subsidence and rapid carbon losses. However, it is unclear how small-scale logging might impact on mangrove functions and services. We experimentally investigated the impact of small-scale tree removal on surface elevation and carbon dynamics in a mangrove forest at Gazi bay, Kenya. The trees in five plots of a Rhizophora mucronata (Lam.) forest were first girdled and then cut. Another set of five plots at the same site served as controls. Treatment induced significant, rapid subsidence (−32.1±8.4 mm yr⁻¹ compared with surface elevation changes of +4.2±1.4 mm yr⁻¹ in controls). Subsidence in treated plots was likely due to collapse and decomposition of dying roots and sediment compaction as evidenced from increased sediment bulk density. Sediment effluxes of CO₂ and CH₄ increased significantly, especially their heterotrophic component, suggesting enhanced organic matter decomposition. Estimates of total excess fluxes from treated compared with control plots were 25.3±7.4 tCO₂ ha⁻¹ yr⁻¹ (using surface carbon efflux) and 35.6±76.9 tCO₂ ha⁻¹ yr⁻¹ (using surface elevation losses and sediment properties). Whilst such losses might not be permanent (provided cut areas recover), observed rapid subsidence and enhanced decomposition of soil sediment organic matter caused by small-scale harvesting offers important lessons for mangrove management. In particular mangrove managers need to carefully consider the trade-offs between extracting mangrove wood and losing other mangrove services, particularly shoreline stabilization, coastal protection and carbon storage.

Adapting to and mitigating against climate change requires the protection and expansion of natural carbon sinks, especially ecosystems with exceptional carbon density such as mangrove forests (an example of ‘blue carbon’). Projects that do this are called ‘nature‐based solutions’ (NbS). International norms regulating NbS stipulate the importance of justice, in contrast with some of the history and practice in wider conservation. However, what justice means and how it manifests in practice remain contentious. Selling carbon credits on the voluntary market is a growing source of funding for NbS. A large literature examines the ethics, economics, science and politics of such payments for ecosystem services (PES), including for blue carbon. The interpretations of justice in this context are particularly contentious, but operational blue carbon projects have not been examined from a justice perspective. Here we report on a case study involving the first blue carbon project, Mikoko Pamoja, and its sister project Vanga Blue Forest, both based in Kenya. We consider how justice is conceived by local participants and beneficiaries, using interviews, focus groups and participant observation to collect data, as well as by international stakeholders and in relevant governing documents and policy. We compare these perceptions with expectations and critiques derived a priori from the literature, including a classic thought experiment that influential justice philosopher John Rawls called the ‘original position’. In contrast to high‐level policy and much of the literature, but in common with Rawls, local stakeholders emphasised distributional aspects of justice. Locally situated interpretations of contentious issues such as elite capture and commodification differed markedly from common interpretations in the literature. Our work emphasises the importance of situating abstract concepts in their local contexts when evaluating justice in NbS projects. It shows how narratives advocating technical precision and economic efficiency in NbS can militate against transparency and agency at a local level and emphasises the critical importance of benefit sharing that is perceived to be fair. Read the free Plain Language Summary for this article on the Journal blog.